(1) Field of the Invention
The present invention relates to a rotary anode for X-ray tubes, and to a method for producing it.
(2) Description of the Related Art
As the rotary anode, hereinafter referred to as target, for X-ray tubes, heretofore use is made of a two-layered structure composed of an X-ray generating layer of a high-melting-point metal of pure tungsten (hereinafter referred to as pure W) or a rhenium-tungsten (hereinafter referred to as Re--W) alloy and an underlying substrate of pure molybdenum (hereinafter referred to as pure Mo) or TZM (this indicates an alloy of 0.5% Ti--0.07% Zr--0.05% C-balance of Mo) as laminated together.
For producing the conventional target, pure W powder or a mixed powder of Re powder and W powder, which is previously mixed with an organic binder to form a material powder. The material powder is put into a mold, and lightly compressed therein from the upper and lower sides. Thereafter, the material powder is stacked with an additional material powder which consists of a predetermined amount of Mo powder or a mixed powder to give a composition of TZM. The additional material powder is previously mixed with an organic substance and, thereafter, put into the mold to form a stacked material body. The stacked material body is compressed therein from the upper and lower sides to give a two-layered disc molding.
Next, the organic substance is removed from it in a hydrogen atmosphere at a temperature falling between 300 and 500.degree. C. Thereafter, the molding is sintered in hydrogen at 1800.degree. C. to form a sintered body. The density of the sintered body generally falls between 90 and 95%.
For increasing its density and for making it have an umbrella-like shape, the sintered body is then subjected to plastic working of, for example, hot rolling and/or hot forging to thereby make it have an umbrella-like shape nearly approaching its final shape, and thereafter this is machined to have a final target shape. In the last step, the thus-shaped target is degassed in vacuum at a temperature of around 1500.degree. C. for the purpose of removing the gaseous component from it. After those steps, the intended target is produced.
However, targets are used under severe conditions, for example, at high temperatures and at high rotating speeds, e.g., at 10,000 rpm. Therefore, the targets are desired to be high quality. In particular, as they shall generate X rays in high vacuum, their life is greatly shortened if the vacuum degree around them is lowered. In addition, if the organic binder used could not be completely removed from them during their production, it remains in them as a carbon residue. In that condition, the targets themselves are heated at high temperatures owing to thermions dashing thereon, and will be dead in a lowered vacuum degree.
Moreover, the conventional process requires long and complicated steps, and also requires expensive raw materials of W, Re and Mo in a large amount of from 3.0 to 4.0 times the weight of the final products. The process thus requiring such a large amount of natural resources and even much energy could not be one that is gentle to the environment. Furthermore, the process could not meet the current requirements, as being uneconomical and expensive.